(USA TODAY) – Deforestation by early farmers likely kicked off an era of man-made climate change long before our present era, suggests a climate scientist taking a hard look at agriculture's early effects.

Chopping down trees with flint axes, planting peas and shearing sheep — those all sound like the prosaic duties of the earliest farmers.

But those same Stone Age sodbusters were likely changing our planet's climate, researchers are now suggesting, long before the greenhouse gas emissions of the industrial era. And that means the "Anthropocene" era, the time of humans making a mark on the planet more striking than natural forces, extends not just to the beginning of the industrial era but to the dawn of agriculture.

How could that be? Mostly because early farmers weren't so good at what they did some 7,000 years ago, suggests environmental scientist Bill Ruddiman of the University of Virginia in Charlottesville.

"How early farmers cleared forests is very different than today. They used a lot more land, and they cleared a lot more forest per farmer," Ruddiman says. In an Annual Review of Earth and Planetary Sciences analysis, Ruddiman finds that archaeology shows climate scientists have underestimated just how many trees early farmers needed to cut down to feed their families. Early farmers didn't plant prairies that needed plows or fertilize the same fields every year. They cut down forests fed by rainfall, moving on to the next one in slash-and-burn fashion every few years after a plot's fertility faded. "All that added up to a lot more forest clearance than climate scientists suspected thousands of years ago," Ruddiman says, with concurrent atmospheric upticks in two important greenhouse gases, carbon dioxide and methane, long before the first fossil-fuel fires.

Greenhouse gasses retain heat, warming the atmosphere. Global warming is mostly the man-made kick to this natural effect provided by adding greenhouse gases to the air through burning fossil fuels.

For more than a decade, most climate scientists marked the starting point of the Anthropocene era as 1850, the point at which these two greenhouse gases began their exponential increase in the atmosphere, up to levels today unseen for at least the past 650,000 years. Those increases are responsible for much of the 1.4-degree Fahrenheit increase in global average atmospheric temperatures during the modern era, according to the American Meteorological Society.

Since 2003 however, Ruddiman has instead argued that the Anthropocene's effects had kicked off global warming much earlier, starting thousands of years ago (the previous period would be the "Holocene," which started 10,000 years ago or so). And more climate scientists are agreeing with him on that today.

"The real question is whether people then really did have global impacts. I'm pretty sure they did," says atmospheric scientist James White of the University of Colorado. "It's a fascinating idea." [more]